The present invention relates to a cannula for puncturing body cavities, comprising an inner and an outer longitudinally cut sleeve with proximal and distal ends. The inner sleeve is arranged within the outer sleeve so as to be longitudinally displaceable and rotatable within the outer sleeve such that the longitudinal cut of the inner sleeve is covered by the outer sleeve in a first rotational position and the longitudinal cuts of the inner and outer sleeves are placed on top of each other in a second rotational position. The distal end of the inner sleeve is provided with a puncture tip that is completely retracted into the outer sleeve if the inner sleeve is in the second rotational position, wherein the inner and outer sleeves and/or the two handle portions are, preferably unreleasably, locked to one another.
